Tuller’s pioneering materials science and engineering research enables better catalytic converters, miniature explosives detectors and thin film microbalances.
MIT Professor Harry L. Tuller and third-year graduate student Thomas Defferriere use a high-temperature micro-probe station to make both electrical and optical measurements of materials for fuel cell electrodes, memory devices and other electroceramic materials. Image, Denis Paiste, MIT Materials Research Laboratory.
MIT Professor Harry L. Tuller and third-year graduate student Thomas Defferriere use a high-temperature micro-probe station to make both electrical and optical measurements of materials for fuel cell electrodes, memory devices and other electroceramic materials. Image, Denis Paiste, MIT Materials Research Laboratory.

What do catalytic converters, miniature explosives detectors and scales for weighing nanoscale quantities have in common? These technologies are all enabled by MIT Professor Harry L. Tuller’s pioneering research on electroceramics, which are complex materials that exhibit a distinctive variety of electrical, optical, magnetic, ionic and electronic properties.

In more than 40 years on the MIT Materials Science and Engineering faculty, Tuller also has mentored many graduate students and postdocs, edited a specialized ceramics journal and co-founded Boston MicroSystems, based on applications of his breakthrough method for micromachining silicon-carbide. His work most often focuses on materials that can operate at high temperatures, for example, in fuel cells and auto exhaust manifolds.

This spring, Tuller, 74, will receive the Thomas Egleston Medal, in recognition of his accomplishments, from his alma mater, Columbia University, where he received both undergraduate and graduate degrees in electrical engineering and a doctorate in solid-state science and engineering.

“I am truly happy to hear that Prof. Tuller will be honored with the Egleston Medal,” says Il-Doo Kim, Professor of Materials Science and Engineering at the Korea Advanced Institute of Science and Technology [KAIST] in the Republic of Korea. Kim worked at MIT as a postdoctoral associate in Tuller’s Crystal Physics and Electroceramics Laboratory from 2003 through 2005, and he continues to collaborate with Tuller on research.

“As the editor-in-chief of the Journal of Electroceramics (since 1997) and a world-leading scientist, Prof. Tuller has accomplished a vast number of fascinating high impact works that contributed significantly to the fields of electroceramics and solid state ionics, that is, defects, electronic structure, and transport of metal oxides and their integration into sensors and fuel cells,” Kim adds. Tuller served as President of the International Society of Solid State Ionics from 2015 to 2017.

Faculty mentor

MIT Professor Bilge Yildiz, who teaches in both the Departments of Nuclear Science and Engineering and Materials Science and Engineering, says she is grateful to Tuller for mentoring her through her own tenure track at MIT and for their continued collaboration. “Professor Tuller is the giant, the most significant figure in the advancement of electroceramics that are enablers to many important technologies including energy conversion and storage, communications, electronics and sensing. Because his work is so fundamental and based on physical principles, his contributions cross-cut and advance all of these wide-ranging applications,” Yildiz notes. Yildiz, Tuller and three international collaborators won the International Union of Materials Research Societies (IUMRS) Somiya Award in 2012 for their work on designing ionic and mixed conducting ceramics for fuel cells.

Avner Rothschild, now a professor of Electrochemical Materials & Devices in the Department of Materials Science and Engineering at the Technion – Israel Institute of Technology, in Haifa, Israel, served as a postdoc under Tuller from 2003 to 2006. “I will always remember the years I spent with Harry Tuller as the most exciting time in my professional career,” Rothschild says.

“Many technologies rely on our ability to tailor the electronic and ionic conductivities in ceramic materials, so-called electroceramic materials. For instance, the combustion process in our cars is controlled by oxygen sensors made of zirconium oxide (ZrO2), and toxic exhaust gases such as carbon monoxide (CO) and nitric oxide (NO) are converted to benign gases such as carbon dioxide (CO2), nitrogen (N2) and oxygen (O2) in the catalytic converter that is made of cerium oxide (CeO2),” Rothschild notes.

MIT Professor Harry L. Tuller will receive the Thomas Egleston Medal on May 30, 2019, from his alma mater, Columbia University, to recognize his pioneering research on electroceramic materials used in catalytic converters, explosives detectors and pico-level microbalances. Photo, Denis Paiste, MIT Materials Research Laboratory.
MIT Professor Harry L. Tuller will receive the Thomas Egleston Medal on May 30, 2019, from his alma mater, Columbia University, to recognize his pioneering research on electroceramic materials used in catalytic converters, explosives detectors and pico-level microbalances. A picometer can detect quantities as tiny as trillionths of a gram. Photo, Denis Paiste, MIT Materials Research Laboratory.

Enabling nanoionics

Tuller pioneered the defect chemistry and ionic transport in cerium oxide, an important material in catalytic converters, gas sensors and fuel cells, while still a graduate student at Columbia University, Rothschild points out. “His doctoral work in Columbia University laid the foundation for understanding how doping, that is, substituting a tiny fraction of one of the constituent elements in an ionic compound with another element, modifies its electrical properties and opens up the way to create new functional materials with tailored electronic and ionic conductivities,” Rothschild says. “For two generations Harry Tuller has been and continues to be the main authority in the field of defect chemistry and ionic and electronic transport in solid-state ionic materials and devices. His research encompasses a vast variety of materials, devices and phenomena, making landmark contributions in many different areas within this field. For example, he pioneered the defect chemistry and ionic transport in nanocrystalline materials, paving the road to a new area of research called nanoionics.”

Tuller’s work is known for its creativity, scientific depth and analytical precision, Rothschild says. Tuller has published more than 480 articles, co-edited 15 books and been awarded 34 patents. The Thomas Egleston Medal for Distinguished Engineering Achievement, awarded annually since 1939, is named for a key founder of the Columbia College School of Mines, which grew into today’s Fu Foundation School of Engineering and Applied Science at Columbia University. Tuller will receive the award in New York City on Thursday, May 30, 2019.

Micromachining silicon carbide

Among his many accomplishments, Tuller says the most exciting was the research developing a micromachining process for diamond-like silicon carbide and shepherding it from lab bench to startup firm Boston MicroSystems through that firm’s acquisition by Pall Corp. in 2013. “This was kind of a fantastic opportunity to start from very fundamental questions in the laboratory, formulate ideas for patents, which were then developed at MIT, and demonstrate that yes, these concepts can be translated into technological innovations, and that they are practical,” Tuller explains.

Tuller’s research in this area was motivated by some fundamental limits of important high-tech materials such as silicon, on the one hand, commonly used for computer chips, because of its ability, in the form of p-n junctions, to switch between states that block or transmit electrical signals (insulating and conductive states), and quartz, on the other hand, used for wristwatches because of its ability to expand or contract in response to an electrical signal (piezoelectric response) and be driven to resonate. Neither material could stand up to the high-temperature environments for which Tuller sought to develop similar applications such as specialized micro-electro-mechanical devices (MEMS). Such MEMS devices tell, for example, a car’s airbag when to turn on, but MEMS have been developed for a wide range of applications from gyroscopes in airplanes to laboratories on a chip.

Silicon loses its semiconducting ability at about 150 degrees Celsius (about 300 degrees Fahrenheit) and it loses its mechanical strength at about 400 degrees Celsius (about 750F), Tuller explains. At about 400 to 500 degrees Celsius, quartz undergoes a destructive transformation in its crystal structure that causes it to shatter. “We discovered a class of very high-temperature piezoelectric materials, that we’ve been refining over a period of years, and now we can replicate at 1,000 degrees Celsius the same level of performance that you can achieve at room temperature,” Tuller says.

“The problem with something which is diamond-like is that it is resistant to nearly every kind of chemical that you could imagine,” Tuller says. “So if you attempt to apply a similar chemical etching route as used in silicon to remove material on an atomic scale, it becomes nearly impossible.” Tuller, and then MIT graduate student Richard F. Mlcak (ScD, 1994), developed a process to micromachine silicon carbide, which is much closer to diamond than to silicon in its ability to withstand high temperatures and retain its semiconducting properties. “So now suddenly, we could essentially replicate everything people could do in silicon in silicon carbide,” Tuller explains.

“And it turns out coincidentally, bringing those two different fields together, that there is a very similar compound to silicon carbide in terms of its crystal structure and lattice parameter and properties, called gallium nitride, which happens to be pieozoelectric. So actually, we’re able to do something which was quite unique, and that is to apply thin layers of gallium nitride to silicon carbide, and thereby cause those MEMS structures to vibrate in a similar way as the quartz crystal monitor, except on a very tiny scale. So now we’re able to make these piezoelectric resonators, working at 10 megahertz, on structures just hundreds of microns in dimensions. We combined those two technologies to form arrays of these sensors and under support from Homeland Security Agency and NASA, we developed a series of devices which could detect explosives and toxic chemicals,” he says. Remarkably, the new MEMS-based sensor, which occupies a chip just a few millimeters in size (about one-eighth inch) can replace large benchtop devices like mass spectrometers that cost about $50,000 each. It became the basis for Boston Microsystems, a pioneer in MEMS designed for harsh environments, and high sensitivity sensors for use in explosives detection and industrial monitoring.

Standing in front of a high-temperature furnace for processing electroceramic materials, MIT Professor Harry L. Tuller holds several samples of metalized thin film metal oxides used in gas sensors, fuel cells and catalytic converters. Tuller’s work is being recognized on May 30 when he will receive the Thomas Egleston Medal from his alma mater, Columbia University. Photo, Denis Paiste, MIT Materials Research Laboratory.
Standing in front of a high-temperature furnace for processing electroceramic materials, MIT Professor Harry L. Tuller holds several samples of metalized thin film metal oxides used in gas sensors, fuel cells and catalytic converters. Tuller’s work is being recognized on May 30 when he will receive the Thomas Egleston Medal from his alma mater, Columbia University. Photo, Denis Paiste, MIT Materials Research Laboratory.

Making connections

The ups and downs of that process enriched his interactions with students, Tuller says. “Something I like about being at MIT is that we’re interested in the connection between fundamental issues and engineering applications. And, I think, to be a good educator at a place like MIT, you have to be able to make those connections, and it’s hard to make those connections unless you’ve had actual experience in the field.”

Taking a new process from laboratory to market means having to deal not just with technical issues, but also with economic issues, Tuller says. “Can you do this in a cost-effective manner? Are there customers who are going to be interested in your product? Can it be manufactured at scale? So suddenly, you have to address all these practical issues and I think, having gone through that process, which sometimes is very painful, I come back in the laboratory, and I think that I can now do a much better job of making those connections for the students,” Tuller says. “And I think that’s one of the things that attracts a lot of students to MIT. It’s not just theory and science, but they know that there are people here who have that excitement about science translated into practice.” In April 2018, Tuller received the Committed to Caring Award at MIT, for which he was nominated by several of his own students.

Electronic nose

Former students and postdocs in Tuller’s group have gone on to positions at top universities and research institutes in Korea, Japan, India, Israel, Switzerland, Austria, Finland, France and Germany. Former Tuller doctoral student Woochul Jung [PhD 2010], participated in the development of an “electronic nose” in Tuller’s lab and is now an Associate Professor of Materials Science and Engineering at the Korea Advanced Institute of Science and Technology. "Professor Tuller is an excellent mentor who always encourages people around him with endless passion and intellectual curiosity,” Jung says. “Above all, he showed me the life of a scientist who enjoys the journey of asking questions and logically finding answers, which has been the most important teaching in my academic career so far."

Tuller is collaborating with his former postdoc Il-Doo Kim at KAIST on extending the “electronic nose” technology to health monitoring. “Here you would just have someone breathe into your device and it would electronically tell you that this person likely has a particular disease, diabetes, or malaria, or even certain kinds of cancer,” Tuller says.

Tuller is now probing how to use color differences as a measurement tool for monitoring defect creation/annihilation and the efficiency of fuel cell electrodes. “A particular material that we’re very much interested in now as a fuel cell electrode is praseodymium cerium oxide. And it turns out this material under normal conditions is quite a deep red. But, as we reduce the oxygen content of the gas phase, it becomes less and less intense red until it becomes transparent. And it turns out over that fairly wide range of gas composition, the intensity of that red can be correlated with the defects, the number of defects, in this material which control its properties,” Tuller says. By monitoring the rate of change in color following a step change in gas composition, one can also characterize the efficiency of the fuel cell electrode and its longer term degradation.

In 2018, the U.S. Department of Energy renewed funding for the Chemomechanics of Far-From-Equilibrium Interfaces (COFFEI) research project, which was its fourth three-year award. The  COFFEI program, a collaboration with five other MIT colleagues, couples chemical and mechanical behavior of electroceramic oxides. Tuller also actively participated in MIT-Skoltech Center for Electrochemical Energy Storage.

Exploiting defects

Defects in an electroceramic material lead to nearly all of its interesting functional properties, whether electronic, optical or magnetic. In energy storage and conversion systems such as batteries and fuel cells, it is the migration of ions that confers on them their electrical properties. “For lithium batteries, it’s lithium ions. For fuel cell materials, it’s usually oxygen or hydrogen ions – protons – that are mobile, and all of those defects are within the crystal lattice,” Tuller explains. “That’s what allows them to actually transport matter, while the electronic properties also depend on the creation of defects in the structure.”

“So being able to identify, monitor and predict how the defect concentrations will depend on the composition of the material, impurities, microstructure, temperature, atmosphere, are all critical in being able to predict and optimize the properties of those materials,” he says. “And that’s something that we spent a lot of time trying to formalize, develop methods to improve characterization and also to model the connection between the defects and the properties of interest in specific applications.” The pico-level microbalances that Tuller developed can detect quantities as tiny as trillionths of a gram.

That quest for structure and order in materials carries over into Tuller’s personal hobbies which include classic sports cars, gardening and photography. He has been a car buff since his college days, and last fall Tuller began restoring a 1972 Jaguar XKE, which he calls one of the most beautiful vehicles ever built. With his research frequently taking him to Japan, Tuller has developed an interest in Japanese gardens. His photography focuses on scenery or architecture. “It’s about form and function, that is what engineering is all about. The same thing as with gardening, the form and the function. There’s something to that,” he says.

Tuller and his wife, Sonia, live in Wellesley, Mass. They have two adult daughters, who also live in greater Boston with their spouses, and three grandchildren.

Since completing his doctorate at Columbia and postdoctoral fellowship at the Technion – Israel Institute of Technology, Tuller has served on the MIT faculty, which he joined in 1975. “I’ve loved being at MIT all these years; it’s actually my first permanent job. The thing I love about MIT is what a stimulating environment it is; but at times I have to limit myself, because it easily becomes overstimulating,” Tuller says.

back to newsletter– Denis Paiste, Materials Research Laboratory
April 26, 2019

 

 

Tuesday, 18 December 2018 22:13

Controllable fast, tiny magnetic bits

MIT researchers show how to make and drive nanoscale magnetic quasi-particles for spintronic memory devices.

For many modern technical applications, such as superconducting wires for magnetic resonance imaging, engineers want to get rid of electrical resistance and its accompanying production of heat as much as possible, but it turns out that a bit of heat production from resistance is a desirable characteristic in metallic thin films for spintronic applications such as solid-state computer memory. Similarly, while defects are often undesirable in materials science, they can be used to control creation of magnetic quasi-particles known as skyrmions.

In separate papers published this month in Nature Nanotechnology and Advanced Materials, both featured on the covers of these journals, researchers in the group of MIT Professor of Materials Science and Engineering Geoffrey S.D. Beach and colleagues in California, Germany, Switzerland and Korea, showed that they can generate stable and fast moving skyrmions in specially formulated layered materials at room temperature, setting world records for size and speed.

Skyrmion Journal Covers Dec 2018 Web
Researchers in the group of MIT Materials Science and Engineering Professor Geoffrey S.D. Beach and colleagues in California, Germany, Switzerland and Korea, graced the covers of Nature Nanotechnology and Advanced Materials this month, with articles showing they can generate stable and fast moving magnetic quasi-particles known as skyrmions at room temperature. Cover images reproduced with permission.

For research published in Advanced Materials, they created a wire that stacks 15 repeating layers of a specially fabricated metal alloy made up of platinum, which is a heavy metal, cobalt-iron-boron, which is a magnetic material, and magnesium-oxygen. In these layered materials, the interface between the platinum metal layer and cobalt-iron-boron creates an environment in which skyrmions can be formed by applying an external magnetic field perpendicular to the film and electric current pulses that travel along the length of the wire.

Notably, under a 20 milliTesla field, a measure of the magnetic field strength, the wire forms skyrmions at room temperature. At temperatures above 349 kelvins (168 degrees Fahrenheit), the skyrmions form without an external magnetic field, an effect caused by the material heating up, and the skyrmions remain stable even after the material is cooled back to room temperature. Previously, results like this had been seen only at low temperature and with large applied magnetic fields, Beach says.

Predictable structure

“After developing a number of theoretical tools, we now can not only predict the internal skyrmion structure and size, but we also can do a reverse engineering problem, we can say, for instance, we want to have a skyrmion of that size, and we’ll be able to generate the multi-layer, or the material, parameters, that would lead to the size of that skyrmion,” says Ivan Lemesh, first author of the Advanced Materials paper and a graduate student in Materials Science and Engineering at MIT. Co-authors include senior author Beach and 17 others in the U.S., Germany, Switzerland, and Korea.

A fundamental characteristic of electrons is their spin, which points either up or down. A skyrmion is a circular cluster of electrons whose spins are opposite to the orientation of surrounding electrons, and the skyrmions maintain a clockwise direction (or counter-clockwise direction). “However, on top of that, we have also discovered that skyrmions in magnetic multilayers develop a complex through-thickness dependent twisted nature,” Lemesh said during a presentation on his work at the Materials Research Society (MRS) fall meeting in Boston on Nov. 30, 2018. Those findings were published in a separate theoretical study in Physical Review B in September.

The current research shows that while this twisted structure of skyrmions has a minor impact on the ability to calculate the average size of the skyrmion, it significantly affects their current-induced behavior.

Fundamental limits

For the Nature Nanotechnology paper, the researchers studied a different magnetic material, layering platinum with a magnetic layer of a gadolinium cobalt alloy, and tantalum oxide. In this material, the researchers showed they could produce skyrmions as small as 10 nanometers and established that they could move at a fast speed in the material.

“What we discovered in this paper is that ferromagnets have fundamental limits for the size of the quasi-particle you can make and how fast you can drive them using currents,” says first author Lucas Caretta, a graduate student in Materials Science and Engineering.

In a ferromagnet, such as cobalt-iron-boron, neighboring spins are aligned parallel to one another and develop a strong directional magnetic moment. To overcome the fundamental limits of ferromagnets, the researchers turned to gadolinium-cobalt, which is a ferrimagnet, in which neighboring spins alternate up and down so they can cancel each other out and result in an overall zero magnetic moment. “One can engineer a ferrimagnet such that the net magnetization is zero, allowing ultrasmall spin textures, or tune it such that the net angular momentum is zero, enabling ultrafast spin textures. These properties can be engineered by material composition or temperature,” Caretta explains.

In 2017, researchers in Beach’s group and their collaborators demonstrated experimentally that they could create these quasi-particles at will in specific locations by introducing a particular kind of defect in the magnetic layer.

“You can change the properties of a material by using different local techniques such as ion bombardment, for instance, and by doing that you change its magnetic properties,” Lemesh says, “and then if you inject a current into the wire, the skyrmion will be born in that location.” Caretta adds, “It was originally discovered with natural defects in the material, then they became engineered defects through the geometry of the wire.” They used this method to create skyrmions in the new Nature Nanotechnology paper.

The researchers made images of the skyrmions in the cobalt-gadolinium mixture at room temperature at synchrotron centers in Germany, using X-ray holography. Felix Büttner, a postdoc in the Beach lab, was one of the developers of this X-ray holography technique. “It’s one of the only techniques that can allow for such highly resolved images where you make out skyrmions of this size,” Caretta says. These skyrmions are as small as 10 nanometers, which is the current world record for room temperature skyrmions. The researchers demonstrated current driven domain wall motion of 1.3 kilometers per second, using a mechanism that can also be used to move skyrmions, which also sets a new world record.

Except for the synchrotron work, all the research was done at MIT. “We grow the materials, do the fabrication and characterize the materials here at MIT,” Caretta says.

Lucas Caretta Ivan Lemesh 9298 DPaiste Web
Lucas Caretta, left, and Ivan Lemesh, both graduate students in the lab of MIT Professor of Materials Science and Engineering Geoffrey S.D. Beach each had a cover article in a peer-reviewed journal article this month. Their work is pioneering new directions for spintronic devices based on quasi-particles known as skyrmions. Caretta and fellow graduate student Mantao Huang created the image for the Nature Nanotechnology cover. Graduate student Ivan Lemesh created the image for the Advanced Materials cover. Photo, Denis Paiste, Materials Research Laboratory.

Magnetic modeling

These skyrmions are one type of spin configuration of electron spins in these materials, while domain walls are another. (Domain walls are the boundary between domains of opposing spin orientation.) In the field of spintronics, these configurations are known as solitons, or spin textures. Since skyrmions are a fundamental property of materials, mathematical characterization of their energy of formation and motion involves a complex set of equations incorporating their circular size, spin angular momentum, orbital angular momentum, electronic charge, magnetic strength, layer thickness, and several special physics terms that capture the energy of interactions between neighboring spins and neighboring layers, such as the exchange interaction.

One of these interactions, which is called the Dzyaloshinskii-Moriya interaction (DMI), is of special significance to forming skyrmions and arises from the interplay between electrons in the platinum layer and the magnetic layer. In the Dzyaloshinskii-Moriya interaction, spins align perpendicular to each other, which stabilizes the skyrmion, Lemesh says. The DMI interaction allows for these skyrmions to be topological, giving rise to fascinating physics phenomena, makes them stable, and allows for them to be moved with a current. “The platinum itself is what provides what’s called a spin current which is what drives the spin textures into motion,” Caretta says. “The spin current provides a torque on the magnetization of the ferro or ferrimagnet adjacent to it, and this torque is what ultimately causes the motion of the spin texture. We’re basically using simple materials to realize complicated phenomena at interfaces.”

In both papers, the researchers performed a mix of micromagnetic and atomistic spin calculations to determine the energy required to form skyrmions and to move them. “It turns out that by changing the fraction of a magnetic layer, you can change the average magnetic properties of the whole system, so now we don’t need to go to a different material to generate other properties, you can just dilute the magnetic layer with a spacer layer of different thickness, and you will wind up with different magnetic properties, and that gives you an infinite number of opportunities to fabricate your system,” Lemesh says.

Precise control

“Precise control of creating magnetic skyrmions is a central topic of the field,” Jiadong Zang, Assistant Professor of Physics at the University of New Hampshire, who was not involved in this research, says regarding the Advanced Materials paper. “This work has presented a new way of generating zero field skyrmions via current pulse. This is definitely a solid step towards skyrmion manipulations in nanosecond regime,” Zang says.

“The fact that the skyrmions are so small but can be stabilized at room temperature makes it very significant,” says Christopher Marrows, Professor of Condensed Matter Physics at the University of Leeds in the United Kingdom, commenting on the Nature Nanotechnology report. Marrows, who also was not involved in this research, noted that the Beach group had predicted room temperature skyrmions in a Scientific Reports paper earlier this year and said the new results are work of the highest quality. “But they made the prediction and real life does not always live up to theoretical expectations, so they deserve all the credit for this breakthrough,” Marrows says.

Commenting on the Nature Nanotechnology paper, Zang adds, “A bottleneck of skyrmion study is to reach a size of smaller than 20 nanometers (the size of state-of-art memory unit), and drive its motion with speed beyond 1 kilometers per second. Both challenges have been tackled in this seminal work.

“A key innovation is to use ferrimagnet, instead of commonly used ferromagnet, to host skyrmions. This work greatly stimulates the design of skyrmion-based memory and logic devices. This is definitely a star paper in the skyrmion field,” Zang says.

Racetrack systems

Solid-state devices built on these skyrmions could someday replace current magnetic storage hard drives. Streams of magnetic skyrmions can act as bits for computer applications. “In these materials, we can readily pattern magnetic tracks,” Beach said during a presentation at MRS.

These new findings could be applied to racetrack memory devices, which were developed by Stuart Parkin at IBM. A key to engineering these materials for use in racetrack devices is engineering deliberate defects into the material where skyrmions can form, because skyrmions form where there are defects in the material. “One can engineer by putting notches in this type of system,” said Beach, who also is co-director of the Materials Research Laboratory (MRL) at MIT. A current pulse injected into the material forms the skyrmions at a notch. “The same current pulse can be used to write and delete,” he said. These skyrmions form extremely quickly, in less than a billionth of a second, Beach says.

“To be able to have a practical operating logic or memory racetrack device, you have to write the bit, so that’s what we talk about in creating the magnetic quasi particle, and you have to make sure that the written bit is very small and you have to translate that bit through the material at a very fast rate,” Caretta says.

“Applications in skyrmion-based spintronics, will benefit, although again it’s a bit early to say for sure what will be the winners among the various proposals, which include memories, logic devices, oscillators and neuromorphic devices,” Leeds professor Marrows notes.

A remaining challenge is the best way to read these skyrmion bits. Work in the Beach group is continuing in this area, Lemesh says, noting that the current challenge is to discover a way to detect these skyrmions electrically in order to use them in computers or phones. “Yea, so you don’t have to take your phone to a synchrotron to read a bit,” Caretta says.

“As a result of some of the work done on ferrimagnets and similar systems called anti-ferromagnets, I think the majority of the field will actually start to shift toward these types of materials because of the huge promise that they hold,” Caretta says.

– Denis Paiste, Materials Research Laboratoryback to newsletter
December 20, 2018

Related:
PAPER: Current-Induced Skyrmion Generation through Morphological Thermal Transitions in Chiral Ferromagnetic Heterostructures
PAPER: Fast current-driven domain walls and smalll skyrmions in a compensated ferrimagnet
PAPER: Twisted domain walls and skyrmions in perpendicularly magnetized multilayers
Fast-moving magnetic particles could enable new form of data storage 

Monday, 23 September 2019 14:09

Controlling 2D magnetism with stacking order

MIT researchers answer puzzling question why magnetism in certain materials is different in atomically thin layers and their bulk forms.
MIT Physics graduate student Dahlia R. Klein, left, and postdoc David MacNeill. Photo, Denis Paiste, Materials Research Laboratory.
MIT Physics graduate student Dahlia R. Klein, left, and postdoc David MacNeill showed that the magnetic order and stacking order are very strongly linked in two-dimensional (2D) magnets such as chromium chloride and chromium iodide, giving engineers a tool to vary the material’s magnetic properties. Photo, Denis Paiste, Materials Research Laboratory.

Researchers led by MIT Physics Professor Pablo Jarillo-Herrero last year showed that rotating layers of hexagonally structured graphene at a particular “magic angle” could change the material’s electronic properties from an insulating state to a superconducting state. Now researchers in the same group and their collaborators have demonstrated that in a different ultra-thin material that also features a honeycomb-shaped atomic structure – chromium trichloride (CrCl3) – they can alter the material’s magnetic properties by shifting the stacking order of layers.

The researchers peeled away two-dimensional (2D) layers of chromium trichloride using tape in the same way researchers peel away graphene from graphite. Then they studied the 2D chromium trichloride’s magnetic properties using electron tunneling. They found that the magnetism is different in 2D and 3D crystals due to different stacking arrangements between atoms in adjacent layers.

At high temperatures, each chromium atom in chromium trichloride has a magnetic moment that fluctuates like a tiny compass needle. Experiments show that as the temperature drops below 14 Kelvin (-434.47 F), deep in the cryogenic temperature range, these magnetic moments freeze into an ordered pattern, pointing in opposite directions in alternating layers (antiferromagnetism). The magnetic direction of all the layers of chromium trichloride can be aligned by applying a magnetic field. But the researchers found that in its 2D form, this alignment needs a magnetic force 10 times stronger than in the 3D crystal. The results were recently published online in Nature Physics.

“What we’re seeing is that it’s 10 times harder to align the layers in the thin limit compared to the bulk, which we measure using electron tunneling in a magnetic field,” says MIT Physics graduate student Dahlia R. Klein, one of the paper’s lead authors. Klein is an NSF Graduate Research Fellow. Physicists call the energy required to align the magnetic direction of opposing layers the interlayer exchange interaction. “Another way to think of it is that the interlayer exchange interaction is how much the adjacent layers want to be anti-aligned,” fellow lead author and MIT postdoc David MacNeill suggests.

The researchers attribute this change in energy to the slightly different physical arrangement of the atoms in 2D chromium chloride. “The chromium atoms form a honeycomb structure in each layer, so it’s basically stacking the honeycombs in different ways,” Klein says. “The big thing is we’re proving that the magnetic and stacking orders are very strongly linked in these materials.”

"Our work highlights how the magnetic properties of 2D magnets can differ very substantially from their 3D counterparts,” says senior author Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics. “This means that we have now a new generation of highly tunable magnetic materials, with important implications for both new fundamental physics experiments and potential applications in spintronics and quantum information technologies."

Layers are very weakly coupled in these materials, known as van der Waals magnets, which is what makes it easy to remove a layer from the 3D crystal with adhesive tape. “Just like with graphene, the bonds within the layers are very strong but there are only very weak interactions between adjacent layers, so you can isolate few-layer samples using tape,” Klein says.

MacNeill and Klein grew the chromium chloride samples, built and tested nanoelectronic devices, and analyzed their results. The researchers also found that as chromium trichloride is cooled from room temperature to cryogenic temperatures, 3D crystals of the material undergo a structural transition that the 2D crystals do not. This structural difference accounts for the higher energy required to align the magnetism in the 2D crystals.

Chromium Trichloride DMacNeill
Bulk single crystal of chromium trichloride, a layered two-dimensional (2D) van der Waals antiferromagnet. Photo, David MacNeill, MIT.

The researchers measured the stacking order of 2D layers through the use of Raman spectroscopy and developed a mathematical model to explain the energy involved in changing the magnetic direction. Co-author and Harvard University postdoc Daniel T. Larson says he analyzed a plot of Raman data that showed variations in peak location with the rotation of the chromium trichloride sample, determining that the variation was caused by the stacking pattern of the layers. “Capitalizing on this connection, Dahlia and David have been able to use Raman spectroscopy to learn details about the crystal structure of their devices that would be very difficult to measure otherwise,” Larson explains. “I think this technique will be a very useful addition to the toolbox for studying ultra-thin structures and devices.” Materials Science and Engineering graduate student Qian Song carried out the Raman spectroscopy experiments in the lab of MIT Assistant Professor of Physics Riccardo Comin. Both also are co-authors of the paper.

“This research really highlights the importance of stacking order on understanding how these van der Waals magnets behave in the thin limit,” Klein says.

MacNeill adds, “The question of why the 2D crystals have different magnetic properties had been puzzling us for a long time. We were very excited to finally understand why this is happening, and it’s because of the structural transition.”

This work builds on two years of prior research into 2D magnets in which Jarillo-Herrero’s group collaborated with researchers at the University of Washington led by Professor Xiaodong Xu, who holds joint appointments in the departments of Materials Science & Engineering, Physics, and Electrical & Computer Engineering, and others. Their work, which was published in a Nature letter in June 2017, showed for the first time that a different material with a similar crystal structure – chromium triiodide (CrI3) – also behaved differently in the 2D form than in the bulk, with few-layer samples showing antiferromagnetism unlike the ferromagnetic 3D crystals.

Jarillo-Herrero’s group went on to show in a May 2018 Science paper that chromium triiodide exhibited a sharp change in electrical resistance in response to an applied magnetic field at low temperature. This work demonstrated that electron tunneling is a useful probe for studying magnetism of 2D crystals. Klein and MacNeill were also the first authors of this paper.

University of Washington Professor Xiaodong Xu says of the latest findings, “The work presents a very clever approach, namely the combined tunneling measurements with polarization resolved Raman spectroscopy. The former is sensitive to the interlayer antiferromagnetism, while the latter is a sensitive probe of crystal symmetry. This approach gives a new method to allow others in the community to uncover the magnetic properties of layered magnets.”

“This work is in concert with several other recently published works,” Xu says. “Together, these works uncover the unique opportunity provided by layered van der Waals magnets, namely engineering magnetic order via controlling stacking order. It is useful for arbitrary creation of new magnetic states, as well as for potential application in reconfigurable magnetic devices.”

Other authors contributing to this work include Efthimious Kaxiras, the John Hasbrouck Van Vleck Professor of Pure and Applied Physics at Harvard University; Harvard graduate student Shiang Fang; Iowa State University Distinguished Professor (Condensed Matter Physics) Paul C. Canfield; Iowa State graduate student Mingyu Xu; and Raquel A. Ribeiro, of Iowa State University and the Federal University of ABC, Santo André, Brazil. This work was supported in part by the Center for Integrated Quantum Materials; the DOE Office of Science, Basic Energy Sciences; the Gordon and Betty Moore Foundation’s EPiQS Initiative; and the Alfred P. Sloan Foundation.

back to newsletterDenis Paiste, Materials Research Laboratory
September 25, 2019

Device made from flexible, inexpensive materials could power large-area electronics, wearables, medical devices, and more.
MIT Wireless Energy Web
Researchers from MIT and elsewhere have designed the first fully flexible, battery-free “rectenna” – a device that converts energy from Wi-Fi signals into electricity – that could be used to power flexible and wearable electronics, medical devices, and sensors for the “Internet of Things.” Illustration, Christine Daniloff, MIT News Office.

Imagine a world where smartphones, laptops, wearables, and other electronics are powered without batteries. Researchers from MIT and elsewhere have taken a step in that direction, with the first fully flexible device that can convert energy from Wi-Fi signals into electricity that could power electronics.

Devices that convert AC electromagnetic waves into DC electricity are known as “rectennas.” The researchers demonstrate a new kind of rectenna, described in a study published Jan. 28, 2019, in Nature, that uses a flexible radio-frequency (RF) antenna that captures electromagnetic waves – including those carrying Wi-Fi – as AC waveforms.
The antenna is then connected to a novel device made out of a two-dimensional semiconductor just a few atoms thick. The AC signal travels into the semiconductor, which converts it into a DC voltage that could be used to power electronic circuits or recharge batteries. 

In this way, the battery-free device passively captures and transforms ubiquitous Wi-Fi signals into useful DC power. Moreover, the device is flexible and can be fabricated in a roll-to-roll process to cover very large areas.

“What if we could develop electronic systems that we wrap around a bridge or cover an entire highway, or the walls of our office and bring electronic intelligence to everything around us? How do you provide energy for those electronics?” says paper co-author Tomás Palacios, a professor in the Department of Electrical Engineering and Computer Science and director of the MIT/MTL Center for Graphene Devices and 2D Systems in the Microsystems Technology Laboratories. “We have come up with a new way to power the electronics systems of the future – by harvesting Wi-Fi energy in a way that’s easily integrated in large areas – to bring intelligence to every object around us.”

Promising early applications for the proposed rectenna include powering flexible and wearable electronics, medical devices, and sensors for the “Internet of Things.” Flexible smartphones, for instance, are a hot new market for major tech firms. In experiments, the researchers’ device can produce about 40 microwatts of power when exposed to the typical power levels of Wi-Fi signals (around 150 microwatts). That’s more than enough power to light up an LED or drive silicon chips.

Another possible application is powering the data communications of implantable medical devices, says co-author Jesús Grajal, a researcher at the Technical University of Madrid. For example, researchers are beginning to develop pills that can be swallowed by patients and stream health data back to a computer for diagnostics.

“Ideally you don’t want to use batteries to power these systems, because if they leak lithium, the patient could die,” Grajal says. “It is much better to harvest energy from the environment to power up these small labs inside the body and communicate data to external computers.”

All rectennas rely on a component known as a “rectifier,” which converts the AC input signal into DC power. Traditional rectennas use either silicon or gallium arsenide for the rectifier. These materials can cover the Wi-Fi band, but they are rigid. And, although using these materials to fabricate small devices is relatively inexpensive, using them to cover vast areas, such as the surfaces of buildings and walls, would be cost-prohibitive. Researchers have been trying to fix these problems for a long time. But the few flexible rectennas reported so far operate at low frequencies and can’t capture and convert signals in gigahertz frequencies, where most of the relevant cell phone and Wi-Fi signals are.

To build their rectifier, the researchers used a novel 2-D material called molybdenum disulfide (MoS2), which at three atoms thick is one of the thinnest semiconductors in the world. In doing so, the team leveraged a singular behavior of MoS2: When exposed to certain chemicals, the material’s atoms rearrange in a way that acts like a switch, forcing a phase transition from a semiconductor to a metallic material. The resulting structure is known as a Schottky diode, which is the junction of a semiconductor with a metal.

“By engineering MoS2 into a 2-D semiconducting-metallic phase junction, we built an atomically thin, ultrafast Schottky diode that simultaneously minimizes the series resistance and parasitic capacitance,” says first author and EECS postdoc Xu Zhang, who will soon join Carnegie Mellon University as an assistant professor.

Parasitic capacitance is an unavoidable situation in electronics where certain materials store a little electrical charge, which slows down the circuit. Lower capacitance, therefore, means increased rectifier speeds and higher operating frequencies. The parasitic capacitance of the researchers’ Schottky diode is an order of magnitude smaller than today’s state-of-the-art flexible rectifiers, so it is much faster at signal conversion and allows it to capture and convert up to 10 gigahertz of wireless signals.

“Such a design has allowed a fully flexible device that is fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others,” Zhang says.

The reported work provides blueprints for other flexible Wi-Fi-to-electricity devices with substantial output and efficiency. The maximum output efficiency for the current device stands at 40 percent, depending on the input power of the Wi-Fi input. At the typical Wi-Fi power level, the power efficiency of the MoS2 rectifier is about 30 percent. For reference, today’s rectennas made from rigid, more expensive silicon or gallium arsenide achieve around 50 to 60 percent.

There are 15 other paper co-authors from MIT, Technical University of Madrid, the Army Research Laboratory, Charles III University of Madrid, Boston University, and the University of Southern California.

The team is now planning to build more complex systems and improve efficiency. The work was made possible, in part, by a collaboration with the Technical University of Madrid through the MIT International Science and Technology Initiatives (MISTI). It was also partially supported by the Institute for Soldier Nanotechnologies, the Army Research Laboratory, the National Science Foundation’s Center for Integrated Quantum Materials, and the Air Force Office of Scientific Research.

back to newsletter–  Rob Matheson | MIT News Office
January 28, 2019

MIT Professor Frances M. Ross is pioneering new techniques to study materials growth and how structure relates to performance.

A hundred years ago, “2d” meant a two-penny, or 1-inch, nail. Today, “2D” encompasses a broad range of atomically thin flat materials, many with exotic properties not found in the bulk equivalents of the same materials, with graphene – the atomically thin form of carbon – perhaps the most prominent. While many researchers at MIT and elsewhere are exploring 2D materials and their special properties, Frances M. Ross, the Ellen Swallow Richards Professor in Materials Science and Engineering, is interested in what happens when these 2D materials and ordinary 3D materials come together.

Research Highlight:
Frances M. Ross

“We’re interested in the interface between a 2D material and a 3D material because every 2D material that you want to use in an application, such as an electronic device, still has to talk to the outside world, which is three-dimensional,” Ross says.

“We’re at an interesting time because there are immense developments in instrumentation for electron microscopy, and there is great interest in materials with very precisely controlled structures and properties, and these two things cross in a fascinating way,” says Ross.

“The opportunities are very exciting,” Ross says. “We’re going to be really improving the characterization capabilities here at MIT.” Ross specializes in examining how nanoscale materials grow and react in both gases and liquid media, by recording movies using electron microscopy. Microscopy of reactions in liquids is particularly useful for understanding the mechanisms of electrochemical reactions that govern the performance of catalysts, batteries, fuel cells, and other important technologies. “In the case of liquid phase microscopy, you can also look at corrosion where things dissolve away, while in gases you can look at how individual crystals grow or how materials react with, say, oxygen,” she says.

Ross joined the Department of Materials Science and Engineering (DMSE) faculty last year, moving from the Nanoscale Materials Analysis department at the IBM Thomas J. Watson Research Center. “I learned a tremendous amount from my IBM colleagues and hope to extend our research in material design and growth in new directions,” she says.

Recording movies

During a recent visit to her lab, Ross explained an experimental set up donated to MIT by IBM. An ultra-high vacuum evaporator system arrived first, to be attached later directly onto a specially designed transmission electron microscope. “This gives powerful possibilities,” Ross explains. “We can put a sample in the vacuum, clean it, do all sorts of things to it such as heating and adding other materials, then transfer it under vacuum into the microscope, where we can do more experiments while we record images. So we can, for example, deposit silicon or germanium, or evaporate metals, while the sample is in the microscope and the electron beam is shining through it, and we are recording a movie of the process.”

While waiting this spring for the transmission electron microscope to be set up, members of Ross’s seven-member research group, including Materials Science and Engineering Postdoc Shu Fen Tan and graduate student Kate Reidy, made and studied a variety of self-assembled structures. The evaporator system was housed temporarily on the fifth level prototyping space of MIT.nano while Ross’s lab was being readied in Building 13. “MIT.nano had the resources and space; we were happy to be able to help,” says Anna Osherov, MIT.nano Assistant Director of User Services.

“All of us are interested in this grand challenge of materials science, which is how do you make a material with the properties you want and, in particular, how do you use nanoscale dimensions to tweak the properties, and create new properties, that you can’t get from bulk materials,” Ross says.

Using the ultra-high vacuum system, graduate student Kate Reidy formed structures of gold and niobium on several 2D materials. “Gold loves to grow into little triangles,” Ross notes. “We’ve been talking to people in physics and materials science about which combinations of materials are the most important to them in terms of controlling the structures and the interfaces between the components in order to give some improvement in the properties of the material,” she notes.

Postdoc Shu Fen Tan synthesized nickel-platinum nanoparticles and examined them using another technique, liquid cell electron microscopy. She could arrange for only the nickel to dissolve, leaving behind spiky skeletons of platinum. “Inside the liquid cell, we are able to see this whole process at high spatial and temporal resolutions,” Tan says. She explains that platinum is a noble metal and less reactive than nickel, so under the right conditions the nickel participates in an electrochemical dissolution reaction and the platinum is left behind.

Platinum is a well-known catalyst in organic chemistry and fuel cell materials, Tan notes, but it is also expensive, so finding combinations with less expensive materials such as nickel is desirable.

“This is an example of the range of materials reactions you can image in the electron microscope using the liquid cell technique,” Ross says. “You can grow materials; you can etch them away; you can look at, for example, bubble formation and fluid motion.”

A particularly important application of this technique is to study cycling of battery materials. “Obviously, I can’t put an AA battery in here, but you could set up the important materials inside this very small liquid cell and then you can cycle it back and forth and ask, if I charge and discharge it 10 times, what happens? It does not work just as well as before - how does it fail?” Ross asks. “Some kind of failure analysis and all the intermediate stages of charging and discharging can be observed in the liquid cell.”

“Microscopy experiments where you see every step of a reaction give you a much better chance of understanding what’s going on,” Ross says.

Moiré patterns

Graduate student Reidy is interested in how to control the growth of gold on 2D materials such as graphene, tungsten diselenide and molybdenum disulfide. When she deposited gold on “dirty” graphene, blobs of gold collected around the impurities. But when Reidy grew gold on graphene that had been heated and cleaned of impurities, she found perfect triangles of gold. Depositing gold on both the top and bottom sides of clean graphene, Reidy saw in the microscope features known as moiré patterns, which are caused when the overlapping crystal structures are out of alignment.

The gold triangles may be useful as photonic and plasmonic structures. “We think this could be important for a lot of applications, and it is always interesting for us to see what happens,” Reidy says. She is planning to extend her clean growth method to form 3D metal crystals on stacked 2D materials with various rotation angles and other mixed layer structures. Reidy is interested in the properties of graphene and hexagonal boron nitride (hBN), as well as two materials that are semiconducting in their 2D single layer form, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2). “One aspect that’s very interesting in the 2D materials community is the contacts between 2D materials and 3D metals,” Reidy says. “If they want to make a semiconducting device or a device with graphene, the contact could be ohmic for the graphene case or a Schottky contact for the semiconducting case, and the interface between these materials is really, really important.”

“You can also imagine devices using the graphene just as a spacer layer between two other materials,” Ross adds.

For device makers, Reidy says it is sometimes important to have a 3D material grow with its atomic arrangement aligned perfectly with the atomic arrangement in the 2D layer beneath. This is called epitaxial growth. Describing an image of gold grown together with silver on graphene, Reidy explains, “We found that silver doesn’t grow epitaxially, it doesn’t make those perfect single crystals on graphene that we wanted to make, but by first depositing the gold and then depositing silver around it, we can almost force silver to go into an epitaxial shape because it wants to conform to what its gold neighbors are doing.”

Electron microscope images can also show imperfections in a crystal such as rippling or bending, Reidy notes. “One of the great things about electron microscopy is that it is very sensitive to changes in the arrangement of the atoms,” Ross says. “You could have a perfect crystal and it would all look the same shade of gray, but if you have a local change in the structure, even a subtle change, electron microscopy can pick it up. Even if the change is just within the top few layers of atoms without affecting the rest of the material beneath, the image will show distinctive features that allow us to work out what’s going on.”

Reidy also is exploring the possibilities of combining niobium - a metal that is superconducting at low temperatures - with a 2D topological insulator, bismuth telluride. Topological insulators have fascinating properties whose discovery resulted in the Nobel Prize in Physics in 2016. “If you deposit niobium on top of bismuth telluride, with a very good interface, you can make superconducting junctions. We’ve been looking into niobium deposition, and rather than triangles we see structures that are more dendritic looking,” Reidy says. Dendritic structures look like the frost patterns formed on the inside of windows in winter or the feathery patterns of some ferns. Changing the temperature and other conditions during the deposition of niobium can change the patterns that the material takes.

All the researchers are eager for new electron microscopes to arrive at MIT.nano to give further insights into the behavior of these materials. “Many things will happen within the next year, things are ramping up already, and I have great people to work with. One new microscope is being installed now in MIT.nano and another will arrive next year. The whole community will see the benefits of improved microscopy characterization capabilities here,” Ross says.

MIT.nano’s Osherov notes that two cryogenic transmission electron microscopes (cryo-TEM) are installed and running. “Our goal is to establish a unique microscopy-centered community. We encourage and hope to facilitate a cross-pollination between the cryo-EM researchers, primarily focused on biological applications, and ‘soft’ material as well as other research communities across campus,” she says. The latest addition of a scanning transmission electron microscope with enhanced analytical capabilities (ultrahigh energy resolution monochromator, 4D STEM detector, Super-X EDS detector, tomography, and several in situ holders) brought in by John Chipman Associate Professor of Materials Science and Engineering James M. LeBeau, once installed, will substantially enhance the microscopy capabilities of the MIT campus. “We consider Professor Ross to be an immense resource for advising us in how to shape the in situ approach to measurements using the advanced instrumentation that will be shared and available to all the researchers within the MIT community and beyond,” Osherov says.

Little drinking straws

“Sometimes you know more or less what you are going to see during a growth experiment, but very often there’s something that you don’t expect,” Ross says. She shows an example of zinc oxide nanowires that were grown using a germanium catalyst. Some of the long crystals have a hole through their centers, creating structures which are like little drinking straws, circular outside but with a hexagonally shaped interior. “This is a single crystal of zinc oxide, and the interesting question for us is why do the experimental conditions create these facets inside, while the outside is smooth?” Ross asks. “Metal oxide nanostructures have so many different applications, and each new structure can show different properties. In particular, by going to the nanoscale you get access to a diverse set of properties.”

“Ultimately, we’d like to develop techniques for growing well-defined structures out of metal oxides, especially if we can control the composition at each location on the structure,” Ross says. A key to this approach is self-assembly, where the material builds itself into the structure you want without having to individually tweak each component. “Self-assembly works very well for certain materials but the problem is that there’s always some uncertainty, some randomness or fluctuations. There’s poor control over the exact structures that you get. So the idea is to try to understand self-assembly well enough to be able to control it and get the properties that you want,” Ross says.

“We have to understand how the atoms end up where they are, then use that self-assembly ability of atoms to make a structure we want. The way to understand how things self-assemble is to watch them do it and that requires movies with high spatial resolution and good time resolution,” Ross explains. Electron microscopy can be used to acquire structural and compositional information and can even measure strain fields or electric and magnetic fields. “Imagine recording all of these things, but in a movie where you are also controlling how materials grow within the microscope. Once you have made a movie of something happening, you analyze all the steps of the growth process and use that to understand which physical principles were the key ones that determined how the structure nucleated and evolved and ended up the way it does.”

Future directions

Ross hopes to bring in a unique high-resolution, high vacuum TEM with capabilities to image materials growth and other dynamic processes. She intends to develop new capabilities for both water-based and gas-based environments. This custom microscope is still in the planning stages but will be situated in one of the rooms in the Imaging Suite in MIT.nano.

“Professor Ross is a pioneer in this field,” Osherov says. “The majority of TEM studies to-date have been static rather than dynamic. With static measurements you are observing a sample at one particular snapshot in time, so you don’t gain any information about how it was formed. Using dynamic measurements, you can look at the atoms hopping from state to state until they find the final position. The ability to observe self-assembling processes and growth in real-time provides valuable mechanistic insights. We’re looking forward to bringing these advanced capabilities to MIT.nano.” she says.

“Once a certain technique is disseminated to the public, it brings attention,” Osherov says. “When results are published, researchers expand their vision of experimental design based on available state-of-the-art capabilities, leading to many new experiments that will be focused on dynamic applications.”

Rooms in MIT.nano feature the quietest space on the MIT campus, designed to reduce vibrations and electromagnetic interference to as low a level as possible. “There is space available for Professor Ross to continue her research and to develop it further,” Osherov says. “The ability of in situ monitoring the formation of matter and interfaces will find applications in multiple fields across campus, and lead to a further push of the conventional electron microscopy limits.”

back to newsletter

Denis Paiste, Materials Research Laboratory
August 26, 2019

 

 

Thursday, 16 August 2018 17:37

DOE funds one-step copper extraction

Allanore lab wins $1.89 million award to advance copper production from sulfur-based minerals using electricity at high temperature.
Copper Extract Sample Ruler 8620 DP
A sample of nearly pure copper deposited on an iron electrode after extraction through an electrochemical process developed by researchers in Associate Professor of Metallurgy Antoine Allanore's lab at MIT. Photo, Denis Paiste, Materials Research Laboratory.

MIT Associate Professor of Metallurgy Antoine Allanore received a $1,893,941 grant from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy [EERE] to run larger scale tests of a new way to produce copper using electricity to separate copper from melted sulfur-based minerals, which are the main source of copper.

One of the primary goals is to make high-purity copper that can go directly into production of copper wire, which is in increasing demand for applications from renewable energy to electric vehicles. Production of electric and hybrid cars and buses is expected to rise from 3.1 million vehicles in 2017 to 27.2 million by 2027, with an accompanying nine-fold increase in demand for copper from 204,000 metric tonnes to 1.9 million metric tonnes [2.09 million U.S. tons] over the same period, according to a March 2017 IDTechEx report commissioned by the International Copper Association (ICA).

In June 2017, researchers in Allanore’s lab identified how to selectively separate pure copper and other metallic elements from sulfide mineral ore in one step. Their molten sulfide electrolysis process eliminates sulfur dioxide, a noxious byproduct of traditional copper extraction methods, instead producing pure elemental sulfur.

“We think that with our technology we could provide these copper wires with less energy consumption and higher productivity,” Allanore says. It may be possible to cut the energy needed for making copper by 20 percent.

In earlier research, Postdoc Sulata K. Sahu and graduate student Brian J. Chmielowiec [’12], decomposed sulfur-rich minerals at high temperature into pure sulfur and extracted three different metals at very high purity: copper, molybdenum, and rhenium. The process is similar to the Hall-Héroult process, which uses electrolysis to produce aluminum, but operates at a higher operating temperature to enable production of liquid copper (melting point of 1,085°C).

Currently, it takes multiple steps to separate out copper, first crushing sulfide minerals, and then floating out the copper-bearing parts. This copper-rich material [copper concentrate] is next partially refined in a smelter, and further purified with electrolytic refining. “Professor Allanore’s approach would work on the copper concentrate and has the potential to produce copper rod in a single operation while separating unwanted impurities and recovering valuable byproducts that are also in the concentrate,” says Hal Stillman, Director of Technology Development and Transfer for the International Copper Association, Ltd. “Professor Allanore’s approach is a big step; it allows a completely new approach to refining copper.”

The three-year, $1.89 million DOE award will allow Allanore’s group to make a larger reactor, producing about 10 times as much liquid copper per hour, and to run the reactor for a longer time, enough to identify what happens to the other metals accompanying copper, which are also commercially important.

Allanore’s group effort began this year, and he hopes it will provide the data needed to move on to a pilot plant within 3 years. “We are aiming to be ready to provide the design criteria, the material and operating conditions of a one metric tonne [about 2,204 pounds, or 1.1 U.S. tons] per day demonstration reactor,” Allanore says. “If everything is successful, that’s what we will deliver.”

Key technical challenges to overcome are proving the durability of the process over a longer time period and verifying the purity of the metals that are made in the process. Some of the byproducts of copper production, selenium, for example, are valuable in their own right.

“The revolution that we are proposing is that only one reactor would do everything. It would make the liquid copper product and allow us to recover elemental sulfur, and allows us to recover selenium,” Allanore says. “We are using electricity, and electrons can be very selective, so we are using electrons in a manner that enables the most efficient separation of the products of the chemical process.”

Conventional pyrometallurgy produces copper by burning the ore in air, requires four steps and produces noxious compounds like sulfur dioxide [SO2] that require secondary processing into sulfuric acid. The initial batch of copper also requires further processing. “It leaves behind copper metal with too much sulfur and too much oxygen, too much for downstream direct wire production,” Allanore says.

Allanore lab’s new molten sulfide electrolysis method better handles trace metals and other impurities that come with the copper, allowing for separation of multiple elements at high purity from the same production process. “Therefore, we can rethink the manufacturing process of copper wires,” Allanore says.

“The essential part is about providing the sector – mining companies, existing smelting companies and existing copper producers – some data that show what happens on longer operations and at a larger scale,” Allanore says.

The International Copper Association conducted a Life Cycle Assessment that identified several areas where the copper industry can improve its environmental footprint. The study indicates the industry needs to continue reducing on-site sulfur dioxide emissions and to get its electricity from sources that are more environmentally friendly. Allanore’s project is relevant to both these issues. “If developed and deployed, it has the potential to decrease energy demand, operate entirely on renewable energy, and reduce sulfur dioxide emissions,” ICA technology director Stillman says. “In addition, it can separate unwanted impurities and recover valuable by-products from the concentrate. Right now, the technical evidence that is creating excitement is a small-scale proof-of-principle demonstration. It’s great that EERE has provided the needed initial funding to explore the potential. If the process works at larger scale, it could be the type of revolutionary approach that the industry is seeking.”

Allanore’s award is one of 24 early-stage, innovative technology projects receiving up to $35 million in support, which DOE’s Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office, announced earlier this year.

back to newsletter– Denis Paiste, Materials Research Laboratory
August 29, 2018

 

Device for harnessing terahertz radiation might enable self-powering implants, cellphones, other portable electronics.
MIT ac dc 02 Web
This schematic figure, from the researchers’ paper, shows a green square that represents graphene on top of a square of another material. The red lines represent terahertz waves. The blue triangles represent antenna that surround the square to capture the terahertz waves and focus the waves to the square. Courtesy of the researchers.

Any device that sends out a Wi-Fi signal also emits terahertz waves — electromagnetic waves with a frequency somewhere between microwaves and infrared light. These high-frequency radiation waves, known as “T-rays,” are also produced by almost anything that registers a temperature, including our own bodies and the inanimate objects around us.

Terahertz waves are pervasive in our daily lives, and if harnessed, their concentrated power could potentially serve as an alternate energy source. Imagine, for instance, a cellphone add-on that passively soaks up ambient T-rays and uses their energy to charge your phone. However, to date, terahertz waves are wasted energy, as there has been no practical way to capture and convert them into any usable form.

Now physicists at MIT have come up with a blueprint for a device they believe would be able to convert ambient terahertz waves into a direct current, a form of electricity that powers many household electronics.

Their design takes advantage of the quantum mechanical, or atomic behavior of the carbon material graphene. They found that by combining graphene with another material, in this case, boron nitride, the electrons in graphene should skew their motion toward a common direction. Any incoming terahertz waves should “shuttle” graphene’s electrons, like so many tiny air traffic controllers, to flow through the material in a single direction, as a direct current.

The researchers have published their results in the journal Science Advances, and are working with experimentalists to turn their design into a physical device.

“We are surrounded by electromagnetic waves in the terahertz range,” says lead author Hiroki Isobe, a postdoc in MIT’s Materials Research Laboratory. “If we can convert that energy into an energy source we can use for daily life, that would help to address the energy challenges we are facing right now.”

Isobe’s co-authors are Liang Fu, the Lawrence C. and Sarah W. Biedenharn Career Development Associate Professor of Physics at MIT; and Su-yang Xu, a former MIT postdoc who is now an assistant professor of chemistry at Harvard University.

Breaking graphene’s symmetry

Over the last decade, scientists have looked for ways to harvest and convert ambient energy into usable electrical energy. They have done so mainly through rectifiers, devices that are designed to convert electromagnetic waves from their oscillating (alternating) current to direct current.

Most rectifiers are designed to convert low-frequency waves such as radio waves, using an electrical circuit with diodes to generate an electric field that can steer radio waves through the device as a DC current. These rectifiers only work up to a certain frequency, and have not been able to accommodate the terahertz range.

A few experimental technologies that have been able to convert terahertz waves into DC current do so only at ultracold temperatures — setups that would be difficult to implement in practical applications.

Instead of turning electromagnetic waves into a DC current by applying an external electric field in a device, Isobe wondered whether, at a quantum mechanical level, a material’s own electrons could be induced to flow in one direction, in order to steer incoming terahertz waves into a DC current.

Such a material would have to be very clean, or free of impurities, in order for the electrons in the material to flow through without scattering off irregularities in the material. Graphene, he found, was the ideal starting material.

To direct graphene’s electrons to flow in one direction, he would have to break the material’s inherent symmetry, or what physicists call “inversion.” Normally, graphene’s electrons feel an equal force between them, meaning that any incoming energy would scatter the electrons in all directions, symmetrically. Isobe looked for ways to break graphene’s inversion and induce an asymmetric flow of electrons in response to incoming energy.

Looking through the literature, he found that others had experimented with graphene by placing it atop a layer of boron nitride, a similar honeycomb lattice made of two types of atoms — boron and nitrogen. They found that in this arrangement, the forces between graphene’s electrons were knocked out of balance: Electrons closer to boron felt a certain force while electrons closer to nitrogen experienced a different pull. The overall effect was what physicists call “skew scattering,” in which clouds of electrons skew their motion in one direction.

Isobe developed a systematic theoretical study of all the ways electrons in graphene might scatter in combination with an underlying substrate such as boron nitride, and how this electron scattering would affect any incoming electromagnetic waves, particularly in the terahertz frequency range.

He found that electrons were driven by incoming terahertz waves to skew in one direction, and this skew motion generates a DC current, if graphene were relatively pure. If too many impurities did exist in graphene, they would act as obstacles in the path of electron clouds, causing these clouds to scatter in all directions, rather than moving as one.

“With many impurities, this skewed motion just ends up oscillating, and any incoming terahertz energy is lost through this oscillation,” Isobe explains. “So we want a clean sample to effectively get a skewed motion.”

MIT ac dc 01 Web
Terahertz waves are pervasive in our daily lives, and if harnessed, their concentrated power could potentially serve as an alternate energy source. Imagine, for instance, a cellphone add-on that passively soaks up ambient T-rays and uses their energy to charge your phone. Illustration, José-Luis Olivares, MIT.

One direction

They also found that the stronger the incoming terahertz energy, the more of that energy a device can convert to DC current. This means that any device that converts T-rays should also include a way to concentrate those waves before they enter the device.

With all this in mind, the researchers drew up a blueprint for a terahertz rectifier that consists of a small square of graphene that sits atop a layer of boron nitride and is sandwiched within an antenna that would collect and concentrate ambient terahertz radiation, boosting its signal enough to convert it into a DC current.

“This would work very much like a solar cell, except for a different frequency range, to passively collect and convert ambient energy,” Fu says.

The team has filed a patent for the new “high-frequency rectification” design, and the researchers are working with experimental physicists at MIT to develop a physical device based on their design, which should be able to work at room temperature, versus the ultracold temperatures required for previous terahertz rectifiers and detectors.

“If a device works at room temperature, we can use it for many portable applications,” Isobe says.

He envisions that, in the near future, terahertz rectifiers may be used, for instance, to wirelessly power implants in a patient’s body, without requiring surgery to change an implant’s batteries. Such devices could also convert ambient Wi-Fi signals to charge up personal electronics such as laptops and cellphones.

“We are taking a quantum material with some asymmetry at the atomic scale, that can now  be utilized, which opens up a lot of possibilities,” Fu says.

This research was funded in part by the U.S. Army Research Laboratory and the U.S. Army Research Office through the Institute for Soldier Nanotechnologies (ISN).

Jennifer Chu | MIT News Office
March 27, 2020

Thursday, 07 December 2017 17:31

Engineers 3-D print a “living tattoo”

New technique 3-D prints programmed cells into living devices for first time. 
MIT 3D Living Tattoo
MIT engineers have devised a 3-D printing technique that uses a new kind of ink made from genetically programmed living cells. Courtesy of the researchers

MIT engineers have devised a 3-D printing technique that uses a new kind of ink made from genetically programmed living cells.

The cells are engineered to light up in response to a variety of stimuli. When mixed with a slurry of hydrogel and nutrients, the cells can be printed, layer by layer, to form three-dimensional, interactive structures and devices.

The team has then demonstrated its technique by printing a “living tattoo” — a thin, transparent patch patterned with live bacteria cells in the shape of a tree. Each branch of the tree is lined with cells sensitive to a different chemical or molecular compound. When the patch is adhered to skin that has been exposed to the same compounds, corresponding regions of the tree light up in response.

The researchers, led by Xuanhe Zhao, the Noyce Career Development Professor in MIT’s Department of Mechanical Engineering, and Timothy Lu, associate professor of biological engineering and of electrical engineering and computer science, say that their technique can be used to fabricate “active” materials for wearable sensors and interactive displays. Such materials can be patterned with live cells engineered to sense environmental chemicals and pollutants as well as changes in pH and temperature.

What’s more, the team developed a model to predict the interactions between cells within a given 3-D-printed structure, under a variety of conditions. The team says researchers can use the model as a guide in designing responsive living materials.

Zhao, Lu, and their colleagues have published their results Dec. 5, 2017,  in the journal Advanced Materials. The paper’s co-authors are graduate students Xinyue Liu, Hyunwoo Yuk, Shaoting Lin, German Alberto Parada, Tzu-Chieh Tang, Eléonore Tham, and postdoc Cesar de la Fuente-Nunez.

A hardy alternative

In recent years, scientists have explored a variety of responsive materials as the basis for 3D-printed inks. For instance, scientists have used inks made from temperature-sensitive polymers to print heat-responsive shape-shifting objects. Others have printed photoactivated structures from polymers that shrink and stretch in response to light.

Zhao’s team, working with bioengineers in Lu’s lab, realized that live cells might also serve as responsive materials for 3D-printed inks, particularly as they can be genetically engineered to respond to a variety of stimuli. The researchers are not the first to consider 3-D printing genetically engineered cells; others have attempted to do so using live mammalian cells, but with little success.

“It turns out those cells were dying during the printing process, because mammalian cells are basically lipid bilayer balloons,” Yuk says. “They are too weak, and they easily rupture.”
Instead, the team identified a hardier cell type in bacteria. Bacterial cells have tough cell walls that are able to survive relatively harsh conditions, such as the forces applied to ink as it is pushed through a printer’s nozzle. Furthermore, bacteria, unlike mammalian cells, are compatible with most hydrogels — gel-like materials that are made from a mix of mostly water and a bit of polymer. The group found that hydrogels can provide an aqueous environment that can support living bacteria.

The researchers carried out a screening test to identify the type of hydrogel that would best host bacterial cells. After an extensive search, a hydrogel with pluronic acid was found to be the most compatible material. The hydrogel also exhibited an ideal consistency for 3-D printing.

“This hydrogel has ideal flow characteristics for printing through a nozzle,” Zhao says. “It’s like squeezing out toothpaste. You need [the ink] to flow out of a nozzle like toothpaste, and it can maintain its shape after it’s printed

Video Abstract for Advanced Materials, 2017, 29, 1704821. Reproduced with permission. ©2017, Wiley-VCH Verlag GmbH & Co. KGaA.

From tattoos to living computers

Lu provided the team with bacterial cells engineered to light up in response to a variety of chemical stimuli. The researchers then came up with a recipe for their 3-D ink, using a combination of bacteria, hydrogel, and nutrients to sustain the cells and maintain their functionality. “We found this new ink formula works very well and can print at a high resolution of about 30 micrometers per feature,” Zhao says. “That means each line we print contains only a few cells. We can also print relatively large-scale structures, measuring several centimeters.”

They printed the ink using a custom 3-D printer that they built using standard elements combined with fixtures they machined themselves. To demonstrate the technique, the team printed a pattern of hydrogel with cells in the shape of a tree on an elastomer layer. After printing, they solidified, or cured, the patch by exposing it to ultraviolet radiation. They then adhere the transparent elastomer layer with the living patterns on it, to skin.

To test the patch, the researchers smeared several chemical compounds onto the back of a test subject’s hand, then pressed the hydrogel patch over the exposed skin. Over several hours, branches of the patch’s tree lit up when bacteria sensed their corresponding chemical stimuli. The researchers also engineered bacteria to communicate with each other; for instance they programmed some cells to light up only when they receive a certain signal from another cell. To test this type of communication in a 3-D structure, they printed a thin sheet of hydrogel filaments with “input,” or signal-producing bacteria and chemicals, overlaid with another layer of filaments of an “output,” or signal-receiving bacteria. They found the output filaments lit up only when they overlapped and received input signals from corresponding bacteria .

Yuk says in the future, researchers may use the team’s technique to print “living computers” — structures with multiple types of cells that communicate with each other, passing signals back and forth, much like transistors on a microchip. “This is very future work, but we expect to be able to print living computational platforms that could be wearable,” Yuk says.

For more near-term applications, the researchers are aiming to fabricate customized sensors, in the form of flexible patches and stickers that could be engineered to detect a variety of chemical and molecular compounds. They also envision their technique may be used to manufacture drug capsules and surgical implants, containing cells engineered produce compounds such as glucose, to be released therapeutically over time.

“We can use bacterial cells like workers in a 3-D factory,” Liu says. “They can be engineered to produce drugs within a 3-D scaffold, and applications should not be confined to epidermal devices. As long as the fabrication method and approach are viable, applications such as implants and ingestibles should be possible.”
This research was supported, in part, by the Office of Naval Research, National Science Foundation, National Institutes of Health, and MIT Institute for Soldier Nanotechnologies. 

Jennifer Chu | MIT News Office
December 5, 2017   

Tuesday, 19 December 2017 14:36

Engineers create plants that glow

Illumination from nanobionic plants might one day replace some electrical lighting.

 

MIT Glowing Plants Web
Illumination of a book (“Paradise Lost,” by John Milton) with the nanobionic light-emitting plants (two 3.5-week-old watercress plants). The book and the light-emitting watercress plants were placed in front of a reflective paper to increase the influence from the light emitting plants to the book pages. Image, Seon-Yeong Kwak

Imagine that instead of switching on a lamp when it gets dark, you could read by the light of a glowing plant on your desk.

MIT engineers have taken a critical first step toward making that vision a reality. By embedding specialized nanoparticles into the leaves of a watercress plant, they induced the plants to give off dim light for nearly four hours. They believe that, with further optimization, such plants will one day be bright enough to illuminate a workspace.

“The vision is to make a plant that will function as a desk lamp — a lamp that you don’t have to plug in. The light is ultimately powered by the energy metabolism of the plant itself,” says Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT and the senior author of the study.

This technology could also be used to provide low-intensity indoor lighting, or to transform trees into self-powered streetlights, the researchers say. MIT postdoc Seon-Yeong Kwak is the lead author of the study, which appears in the journal Nano Letters.

Nanobionic plants

Plant nanobionics, a new research area pioneered by Strano’s lab, aims to give plants novel features by embedding them with different types of nanoparticles. The group’s goal is to engineer plants to take over many of the functions now performed by electrical devices. The researchers have previously designed plants that can detect explosives and communicate that information to a smartphone, as well as plants that can monitor drought conditions.

Lighting, which accounts for about 20 percent of worldwide energy consumption, seemed like a logical next target. “Plants can self-repair, they have their own energy, and they are already adapted to the outdoor environment,” Strano says. “We think this is an idea whose time has come. It’s a perfect problem for plant nanobionics.”

To create their glowing plants, the MIT team turned to luciferase, the enzyme that gives fireflies their glow. Luciferase acts on a molecule called luciferin, causing it to emit light. Another molecule called co-enzyme A helps the process along by removing a reaction byproduct that can inhibit luciferase activity.

The MIT team packaged each of these three components into a different type of nanoparticle carrier. The nanoparticles, which are all made of materials that the U.S. Food and Drug Administration classifies as “generally regarded as safe,” help each component get to the right part of the plant. They also prevent the components from reaching concentrations that could be toxic to the plants.

The researchers used silica nanoparticles about 10 nanometers in diameter to carry luciferase, and they used slightly larger particles of the polymers PLGA and chitosan to carry luciferin and coenzyme A, respectively. To get the particles into plant leaves, the researchers first suspended the particles in a solution. Plants were immersed in the solution and then exposed to high pressure, allowing the particles to enter the leaves through tiny pores called stomata.

Particles releasing luciferin and coenzyme A were designed to accumulate in the extracellular space of the mesophyll, an inner layer of the leaf, while the smaller particles carrying luciferase enter the cells that make up the mesophyll. The PLGA particles gradually release luciferin, which then enters the plant cells, where luciferase performs the chemical reaction that makes luciferin glow.

Video: Melanie Gonick/MIT

The researchers’ early efforts at the start of the project yielded plants that could glow for about 45 minutes, which they have since improved to 3.5 hours. The light generated by one 10-centimeter watercress seedling is currently about one-thousandth of the amount needed to read by, but the researchers believe they can boost the light emitted, as well as the duration of light, by further optimizing the concentration and release rates of the components.

Plant transformation

Previous efforts to create light-emitting plants have relied on genetically engineering plants to express the gene for luciferase, but this is a laborious process that yields extremely dim light. Those studies were performed on tobacco plants and Arabidopsis thaliana, which are commonly used for plant genetic studies. However, the method developed by Strano’s lab could be used on any type of plant. So far, they have demonstrated it with arugula, kale, and spinach, in addition to watercress.

For future versions of this technology, the researchers hope to develop a way to paint or spray the nanoparticles onto plant leaves, which could make it possible to transform trees and other large plants into light sources.

“Our target is to perform one treatment when the plant is a seedling or a mature plant, and have it last for the lifetime of the plant,” Strano says. “Our work very seriously opens up the doorway to streetlamps that are nothing but treated trees, and to indirect lighting around homes.”

The researchers have also demonstrated that they can turn the light off by adding nanoparticles carrying a luciferase inhibitor. This could enable them to eventually create plants that shut off their light emission in response to environmental conditions such as sunlight, the researchers say.

The research was funded by the U.S. Department of Energy.

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Anne Trafton | MIT News Office 
December 12, 2017

 

Monday, 24 June 2019 16:02

Enriching solid-state batteries

MIT researchers show method to make a lithium-rich ceramic electrolyte that is smaller, safer and faster.
MIT Associate Professor Jennifer Rupp stands in front of a pulsed laser deposition chamber. Photo, Denis Paiste, MIT MRL.
MIT Associate Professor Jennifer Rupp stands in front of a pulsed laser deposition chamber in which her team developed a new lithium garnet electrolyte material with the fastest reported ionic conductivity of its type. The new technique pioneered by Rupp and her colleagues produces a thin film that is about 330 nanometers thick. “Having the lithium electrolyte as a solid-state very fast conductor allows you to dream out loud of anything else you can do with fast lithium motion,” Rupp says. Photo, Denis Paiste, Materials Research Laboratory.

Researchers at MIT have come up with a new pulsed laser deposition technique to make thinner lithium electrolytes using less heat, that promises faster charging and potentially higher voltage solid-state lithium ion batteries.

Key to the new technique for processing the solid-state battery electrolyte is alternating layers of the active electrolyte lithium garnet component (chemical formula, Li6.25Al0.25La3Zr2O12, or LLZO), with layers of lithium nitride (chemical formula, Li3N). First, these layers are built up like a wafer cookie using a pulsed laser deposition process at about 300 degrees Celsius (572 degrees Fahrenheit). Then they are heated to 660 C and slowly cooled, a process known as annealing.

During the annealing process, nearly all of the nitrogen atoms burn off into the atmosphere and the lithium atoms from the original nitride layers fuse into the lithium garnet forming a single lithium-rich, ceramic thin film. The extra lithium content in the garnet film allows the material to retain the cubic structure needed for positively charged lithium ions (cations) to move quickly through the electrolyte. The findings were reported in a Nature Energy paper published online May 20, 2019, by MIT Associate Professor Jennifer L. M. Rupp and her students Reto Pfenninger, Michal M. Struzik, Inigo Garbayo and collaborator Evelyn Stilp.

“The really cool new thing is that we found a way to bring the lithium into the film at deposition by using lithium nitride as an internal lithiation source,” senior author Rupp says. Rupp holds joint appointments at MIT in the Materials Science and Engineering department and the Electrical Engineering and Computer Science department.

“The second trick to the story is that we use lithium nitride, which is close in bandgap to the laser that we use in the deposition, whereby we have a very fast transfer of the material, which is another key factor to not lose lithium to evaporation during a pulsed laser deposition,” Rupp explains.

Safer technology

Lithium batteries with commonly used electrolytes made by combining a liquid and a polymer can pose a fire risk when the liquid is exposed to air. Solid-state batteries are desirable because they replace the commonly used liquid polymer electrolytes in consumer lithium batteries with a solid material that is safer. “So we can kick that out, bring something safer in the battery, and decrease the electrolyte component in size by a factor of 100 by going from the polymer to the ceramic system,” Rupp explains.

Although other methods to produce lithium-rich ceramic materials on larger pellets or tapes, which are heated using a process called sintering, can yield a dense microstructure that retains a high lithium concentration, they require higher heat and result in bulkier material. The new technique pioneered by Rupp and her students produces a thin film that is about 330 nanometers thick (less than 1.5 hundred-thousandths of an inch). “Having a thin film structure instead of a thick ceramic is attractive for battery electrolyte in general because it allows you to have more volume in the electrodes where you want to have the active storage capacity. So the holy grail is be thin and be fast,” she says.

Compared to the classic ceramic coffee mug, which under high magnification shows metal oxide particles with a grain size of 10s to 100s of microns, the lithium (garnet) oxide thin films processed using Rupp’s methods show nanometer scale grain structures that are 1,000 to 10,000 times smaller. That means Rupp can engineer thinner electrolytes for batteries. “There is no need in a solid-state battery to have a large electrolyte,” she says.

Faster ionic conduction

Instead what is needed is an electrolyte with faster conductivity. The unit of measurement for lithium ion conductivity is expressed in Siemens. The new multilayer deposition technique produces a lithium garnet (LLZO) material that shows the fastest ionic conductivity yet for a lithium-based electrolyte compound, about 2.9 x 10-5 Siemens (0.000029 Siemens) per centimeter. This ionic conductivity is competitive with solid-state lithium battery thin film electrolytes based on LIPON (lithium phosphorus oxynitride electrolytes) and adds a new film electrolyte material to the landscape.

“Having the lithium electrolyte as a solid-state very fast conductor allows you to dream out loud of anything else you can do with fast lithium motion,” Rupp says.

A battery’s negatively charged electrode stores power. The work points the way toward higher voltage batteries based on lithium garnet electrolytes both because its lower processing temperature opens the door to using materials for higher voltage cathodes that would be unstable at higher processing temperatures, and its smaller electrolyte size allows physically larger cathode volume in the same battery size.

Co-authors Michal Struzik and Reto Pfenninger carried out processing and Raman spectroscopy measurements on the lithium garnet material. These measurements were key to showing the material’s fast conduction at room temperature as well as understanding the evolution of its different structural phases.

“One of the main challenges in understanding the development of the crystal structure in LLZO was to develop appropriate methodology. We have proposed a series of experiments to observe development of the crystal structure in the (LLZO) thin film from disordered or 'amorphous' phase to fully crystalline, highly conductive phase utilizing Raman Spectroscopy upon thermal annealing under controlled atmospheric conditions,” says co-author Struzik, who was a postdoc working at ETH and MIT with Rupp’s group, and is now a professor at Warsaw University of Technology in Poland. “That allowed us to observe and understand, how the crystal phases are developed and, as a consequence, the ionic conductivity improved,” he explains.

Their work shows that during the annealing process, lithium garnet evolves from the amorphous phase in the initial multilayer processed at 300 C through progressively higher temperatures to a low conducting tetragonal phase in a temperature range from about 585 C to 630 C and to the desired highly conducting cubic phase after annealing at 660 C. Notably, this temperature of 660 C to achieve the highly conducting phase in the multilayer approach is nearly 400 C lower than the 1,050 C needed to achieve it with prior sintering methods using pellets or tapes.

“One of the greatest challenges facing the realization of solid-state batteries lies in the ability to fabricate such devices. It is tough to bring the manufacturing costs down to meet commercial targets that are competitive with today's liquid electrolyte based lithium-ion batteries, and one of the main reasons is the need to use high temperatures to process the ceramic solid electrolytes,” says Professor Peter Bruce, Wolfson Chair of the Department of Materials at the University of Oxford, who was not involved in this research.

Optical microscopy image of a solid lithium garnet (LLZO) thin film electrolyte deposited on a magnesium oxide. Reprinted with permission of Nature Energy.
Optical microscopy image of a solid lithium garnet (LLZO) thin film electrolyte deposited on a magnesium oxide (MgO) substrate. Reproduced with permission of Nature Energy.

“This important paper reports a novel and imaginative approach to addressing this problem by reducing the processing temperature of garnet-based solid-state batteries by more than half, that is, by hundreds of degrees,” Bruce adds. “Normally high temperatures are required to achieve sufficient solid-state diffusion to intermix the constituent atoms of ceramic electrolyte. By interleaving lithium layers in an elegant nanostructure the authors have overcome this barrier.”

After demonstrating the novel processing and high conductivity of the lithium garnet electrode, the next step will be to test the material in an actual battery to explore how the material reacts with a battery cathode and how stable it is. “There is still a lot to come,” Rupp predicts.

Understanding aluminum dopant sites

A small fraction of aluminum is added to the lithium garnet formulation because aluminum is known to stabilize the highly conductive cubic phase in this high-temperature ceramic. The researchers complemented their Raman spectroscopy analysis with another technique, known as negative-ion time-of-flight secondary ion mass spectrometry (TOF-SIMS), which shows that the aluminum retains its position at what were originally the interfaces between the lithium nitride and lithium garnet layers before the heating step expelled the nitrogen and fused the material.

“When you look at large scale processing of pellets by sintering, then everywhere where you have a grain boundary, you will find close to it a higher concentration of aluminum. So we see a replica of that in our new processing but on a smaller scale at the original interfaces,” Rupp says. “These little things are what adds up also not only to my excitement in engineering but my excitement as a scientist to understand phase formations, where that goes and what that does,” Rupp says.

“Negative TOF-SIMS was indeed challenging to measure since it is more common in the field to perform this experiment with focus on positively charged ions,” explains Pfenninger, who worked at ETH and MIT with Rupp’s group. “However, for the case of the negatively charged nitrogen atoms we could only track it in this peculiar setup. The phase transformations in thin films of LLZO have so far not been investigated in temperature-dependent Raman spectroscopy – another insight towards the understanding thereof.”

The paper’s other authors are Inigo Garbayo, who is now at CIC EnergiGUNE in Minano, Spain, and Evelyn Stilp, who was then with Empa, Swiss Federal Laboratories for Materials Science and Technology, in Dubendorf, Switzerland.

Rupp began this research while serving as a Professor of Electrochemical Materials at ETH Zurich (the Swiss Federal Institute of Technology) before she joined the MIT faculty in February 2017. MIT and ETH have jointly filed for two patents on the multi-layer lithium garnet/lithium nitride processing. This new processing method, which allows precise control of lithium concentration in the material, can also be applied to other lithium oxide films such as lithium titanate and lithium cobaltate that are used in battery electrodes. “That is something we invented. That’s new in ceramic processing,” Rupp says.

“It is a smart idea to use Li3N as a lithium source during preparation of the garnet layers, as lithium loss is a critical issue during thin film preparation otherwise,” comments University Professor Jürgen Janek at Justus Liebig University Giessen in Germany. Janek, who was not involved in this research, adds that “the quality of the data and the analysis is convincing.”

“This work is an exciting first step in preparing one of the best oxide-based solid electrolytes in an intermediate temperature range,” Janek says. “It will be interesting to see, whether the intermediate temperature of about 600 °C is sufficient to avoid side reactions with the electrode materials.”

Oxford Prof. Bruce notes the novelty of the approach, adding “I'm not aware of similar nanostructured approaches to reduce diffusion lengths in solid-state synthesis.”

“Although the paper describes specific application of the approach to the formation of lithium rich and therefore highly conducting garnet solid electrolytes, the methodology has more general applicability and therefore significant potential beyond the specific examples provided in the paper,” Bruce says. Commercialization may be needed to be demonstrate this approach at larger scale, he suggests.

While the immediate impact of this work is likely to be on batteries, Rupp predicts another decade of exciting advances based on applications of her processing techniques to devices for neuromorphic computing, artificial intelligence and fast gas sensors. “The moment the lithium is in a small solid-state film, you can use the fast motion to trigger other electrochemistry,” she says.

Several companies have already expressed interest in using the new electrolyte approach. “It’s good for me to work with strong players in the field so they can push out the technology faster than anything I can do,” Rupp says.

This work was funded by the MIT Lincoln Laboratory, the Thomas Lord Foundation, Competence Center Energy and Mobility, and Swiss Electrics.

back to newsletter Denis Paiste, Materials Research Laboratory
June 27, 2019

 

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